Rollup merge of #102350 - TaKO8Ki:incomplete-fn-in-struct-definition, r=fee1-dead

Improve errors for incomplete functions in struct definitions

Given the following code:

```rust
fn main() {}

struct Foo {
    fn
}
```

[playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=29139f870511f6918324be5ddc26c345)

The current output is:

```
   Compiling playground v0.0.1 (/playground)
error: functions are not allowed in struct definitions
 --> src/main.rs:4:5
  |
4 |     fn
  |     ^^
  |
  = help: unlike in C++, Java, and C#, functions are declared in `impl` blocks
  = help: see https://doc.rust-lang.org/book/ch05-03-method-syntax.html for more information

error: could not compile `playground` due to previous error
```

In this case, rustc should suggest escaping `fn` to use it as an identifier.

compiler/rustc_parse/src/parser/item.rs

@@ -1753,18 +1753,24 @@ impl<'a> Parser<'a> {
                 };
                 // We use `parse_fn` to get a span for the function
                 let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
-                if let Err(mut db) =
-                    self.parse_fn(&mut AttrVec::new(), fn_parse_mode, lo, &inherited_vis)
-                {
-                    db.delay_as_bug();
-                }
+                match self.parse_fn(&mut AttrVec::new(), fn_parse_mode, lo, &inherited_vis) {
+                    Ok(_) => {
                         let mut err = self.struct_span_err(
                             lo.to(self.prev_token.span),
                             &format!("functions are not allowed in {adt_ty} definitions"),
                         );
-                err.help("unlike in C++, Java, and C#, functions are declared in `impl` blocks");
+                        err.help(
+                            "unlike in C++, Java, and C#, functions are declared in `impl` blocks",
+                        );
                         err.help("see https://doc.rust-lang.org/book/ch05-03-method-syntax.html for more information");
                         err
+                    }
+                    Err(err) => {
+                        err.cancel();
+                        self.restore_snapshot(snapshot);
+                        self.expected_ident_found()
+                    }
+                }
             } else if self.eat_keyword(kw::Struct) {
                 match self.parse_item_struct() {
                     Ok((ident, _)) => {

src/test/ui/parser/fn-field-parse-error-ice.rs

@@ -3,7 +3,7 @@
 struct Baz {
     inner : dyn fn ()
     //~^ ERROR expected `,`, or `}`, found keyword `fn`
-    //~| ERROR functions are not allowed in struct definitions
+    //~| ERROR expected identifier, found keyword `fn`
     //~| ERROR cannot find type `dyn` in this scope
 }
 

src/test/ui/parser/fn-field-parse-error-ice.stderr

@@ -4,16 +4,18 @@ error: expected `,`, or `}`, found keyword `fn`
 LL |     inner : dyn fn ()
    |                ^ help: try adding a comma: `,`
 
-error: functions are not allowed in struct definitions
+error: expected identifier, found keyword `fn`
   --> $DIR/fn-field-parse-error-ice.rs:4:17
    |
 LL | struct Baz {
    |        --- while parsing this struct
 LL |     inner : dyn fn ()
-   |                 ^^
+   |                 ^^ expected identifier, found keyword
    |
-   = help: unlike in C++, Java, and C#, functions are declared in `impl` blocks
-   = help: see https://doc.rust-lang.org/book/ch05-03-method-syntax.html for more information
+help: escape `fn` to use it as an identifier
+   |
+LL |     inner : dyn r#fn ()
+   |                 ++
 
 error[E0412]: cannot find type `dyn` in this scope
   --> $DIR/fn-field-parse-error-ice.rs:4:13

src/test/ui/structs/incomplete-fn-in-struct-definition.rs

@@ -0,0 +1,5 @@
+fn main() {}
+
+struct S {
+    fn: u8 //~ ERROR expected identifier, found keyword `fn`
+}

src/test/ui/structs/incomplete-fn-in-struct-definition.stderr

@@ -0,0 +1,15 @@
+error: expected identifier, found keyword `fn`
+  --> $DIR/incomplete-fn-in-struct-definition.rs:4:5
+   |
+LL | struct S {
+   |        - while parsing this struct
+LL |     fn: u8
+   |     ^^ expected identifier, found keyword
+   |
+help: escape `fn` to use it as an identifier
+   |
+LL |     r#fn: u8
+   |     ++
+
+error: aborting due to previous error
+